Hokkaido, the northernmost main island, was formed by which the Kuril
Arc collided with the Northeast Japan Arc at the central part of
Hokkaido after the Middle Miocene. Therefore, Hokkaido is divided into
three parts: the eastern region (part of the Northeast Japan Arc), the
central region (an arc-arc collision zone), and the western region
(part of the Kuril Arc). Two island arcs meet each other in the central
region but the region is not a plate boundary zone. It is thought that
there was a plate boundary in the central region and it shifted to the
margin of the Sea of Japan in the end of the Tertiary or the Early
Quaternary. The volcanic front runs nearly east-west from the eastern
region through the central region and turns to the south in the western
region. This front line is parallel to the Kuril Trench and the Japan
Hokkaido has broad gentle landforms and less mountainous areas compared to Honshu, Shikoku, and Kyushu. The mountains in Hokkaido generally have gentle slopes in the northern part and steep slopes in the southern part.
For the basement rocks (geotectonic subdivisions), see also “Outline of landforms and geology of Japan”.
In the eastern region, northeastward trending uplift areas with
volcanoes are arranged in echelon toward the Kuril Islands, including
the Akan volcanoes, the Shiretoko Peninsula, the Kunasir Island, and the
Etorofu Island. The en echelon arrangement of the uplift areas is
attributed to the oblique subduction of the Pacific Plate at the Kuril
Trench. The Konsen Plateau, the Kushiro Plain, and the Tokachi Plain
spread in the Kuril outer arc, on the south of the volcanic area.
Massive volcanic products and clastic material produced by erosion
transferred from the volcanoes onto the Konsen Plateau and the Kushiro
Plain to form plateaus and hills. This sediment covers Cretaceous and
Paleogene rocks. The Shiranuka Hills situated between the Kushiro Plain
and the Tokachi Plain are a low relief uplift area. Mountains in the
central part of the Shiranuka Hills comprise Cretaceous and Paleogene
rocks, and they are surrounded by hills consisting of Neogene and
Quaternary formations. Pliocene-Quaternary formations are distributed in
the Tokachi Plain.
The Kitami Mountains are placed in the Kuril inner arc, to the east of the Teshio Mountains. This mountain range is the stablest area in Japan, in which active faulting and earthquakes rarely occur. The Kitami Mountains consist mainly of Cretaceous-Paleogene basement rocks and volcanic rocks that erupted in the Miocene or later and covered the basement. The volcanic rocks formed lava plateaus. The Cretaceous-Paleogene rocks in the Hidaka Supergroup consist of turbidite sandstone and mudstone, and mélange including chert, limestone and greenstone (accretionary complex).
The Kussharo volcano in the eastern region is accompanied with the largest caldera (26 × 20 km) in Japan.
The Teshio Mountains and the Yubari Mountains are located in the central region, both of which have north-south main ridges. A depression zone is placed between the Teshio-Yubari Mountains and the Kitami-Yubari Mountains. The Teshio Mountains consist of Cretaceous-Tertiary rocks, produced by folding with faults. The Yubari Mountains have Jurassic-Cretaceous rocks (Sorachi Group consisting of greenstone [including basaltic pillow lava, hyaloclastite, and diabase], chert, micrite limestone, and sandstone with felsic tuff) and serpentinite in and around the main ridge. Cretaceous forearc sediments (Yezo Supergroup) are distributed around the Jurassic-Cretaceous rocks and Paleogene sediments with interbedded coal seams are found on the west of the Cretaceous sediments. These mountains were upheaved by the collision of the two island arcs.
The Hidaka Mountains in the western margin of the Kuril outer arc are
situated to the southeast of the Yubari Mountains. This mountain range
was also uplifted in association with the colliding Kuril Arc. In the
Hidaka Mountains, the Hidaka metamorphic rocks, the Cretaceous-Paleogene
system, and the Cretaceous system are distributed in the central part,
eastern part, and western part, respectively (Figure). The crust of the
island arc and the upper mantle were thrusted up on the metamorphosed
oceanic crust along the Hidaka thrust fault. The Hidaka metamorphic
zone, therefore, is regarded as the exposed cross section of the island
arc crust. High pressure type metamorphic rock zone (Kamuikotan Belt)
is found on the west of the Hidaka metamorphic zone.
The Daisetsu and Tokachi volcanic area (Ishikari Mountains) between the Hidaka Mountains and the Kitami Mountains are the highest elevation area in Hokkaido (the highest peak is 2290 m high).
The Ishikari Plain is an alluvial lowland to the west of the Yubari
Mountains. The Oshima Peninsula is located on the west of the Ishikari
Plain, which is part of the Northeast Japan Arc. Mountains, volcanoes,
and plains in the peninsula are distributed complicatedly. This
irregular arrangement is a different feature from the zonal
arrangements of landforms in the central and eastern regions. A
Jurassic accretionary complex intruded by Cretaceous granite is found
in this region, which is uncoformably covered with Neogene-Quaternary
volcanic rocks and sediments.
Some volcanoes such as the Shikotsu volcano and the Toya volcano in this region have large calderas and pyroclastic plateaus.